It more particularly applies to simple displays permitting the display of fixed images or pictures, as well as the production of complex, multiplexed screens permitting the display of moving pictures, e.g. of the television picture type.
It is of particular interest for field emission-excited cathodoluminescence display means, comprising a microtip emissive cathode electron source. Display means of this type are e.g. described in FR-A-2 687 839, FR-A-2 633 763 and U.S. Pat. No. 5,231,387.
FIG. 1 shows a display of this type having a microtip electron source 1, which has the insulating substrate 1a, resistive layer 1b, microtips 1c, insulating layer 1d and a grid 1e. A space 3 in which is formed a vacuum separates said microtip source 1 from an electrically insulating, transparent substrate 5 provided with an electrically conductive, transparent layer forming an anode. On said anode is provided, facing the microtip source, a layer 9 of a cathodoluminescent material also known as a phosphor.
Under the impact of the electrons emitted by the microtips 1c when the source is functioning, said layer 8 emits a light 11, which a screen user 13 observes through the transparent substrate 5. Therefore with said screen the phosphor is observed from the side opposite to its excitation.
In the case of microtip flat screens, the electrons are accelerated at very low voltages (a few hundred volts) compared with the voltages used in cathode screens (20 to 25 kV). Therefore, in such screens, the phosphors are essentially surface excited and must have a minimum pollution. A contamination or the presence of a thin layer or film on the surface of the phosphor grains will lead to a reduction in the light output. Therefore the quality of the deposited phosphor layer is an important parameter.
In the case of flat microtip screens, the deposition of the phosphor layer must take place on a transparent, insulating substrate having a transparent, conductive coating, e.g. of indium and tin oxide (ITO). Moreover, for colour display, the screen must have three groups of conductor tracks having a width of a few dozen micrometers and on which will be respectively deposited the phosphors corresponding to red, blue and green.
To obtain a satisfactory quality, the different deposited layers must have the following properties: good adhesion to the substrate, thickness homogeneity, high degree of purity for avoiding mixtures of colours and composition adapted to the deposition mode. In addition, the deposition procedure must be reproducible, easy to perform and appropriate for high mass production rates.
Another important condition which must be fulfilled is that the transparent, conductive coating forming the conductors, e.g. of indium and tin oxide, must withstand the phosphor deposition conditions, i.e. must maintain its transparency and electrical conductivity.
A widely used procedure for producing deposits of this type is electrophoresis, according to which the phosphor particles are deposited from a suspension thereof, under the action of an electric field, the part to be coated serving as the anode (anaphoresis) or cathode (cataphoresis).
FR-A-2 532 957 describes non-aqueous suspensions permitting the deposition of phosphor powders by anaphoresis. The liquid phase of the suspension used is constituted by a ketone-based solvent containing a dispersing agent constituted by nitrocellulose, as well as a strong acid and a strong base for giving the suspension an adequate conductivity. With such suspensions the deposition time is approximately 0.3 sec.
The use of this procedure makes it possible to avoid a deterioration of the conductive, transparent layer of the deposition electrode, because the latter takes place by anaphoresis using the substrate in the anode position, which avoids the appearance of H.sup.+ ions on the electrode. However, as the deposition period is very short, it is not possible to guarantee the obtaining of a reproducible, homogeneous thickness of the deposited layers. Thus, the light outputs of the screens are not reproducible and the appearance is not uniform.
U.S. Pat. No. 3,714,011 describes a procedure for depositing phosphors by cataphoresis. The suspension used is a non-aqueous suspension for avoiding the problems linked with the present of H.sup.+ ions on the electrode. Such a suspension can be constituted by an organic solvent containing a water-miscible, cathodic depolarizing agent, such as acetone, a little water and a metal salt such as hydrogenated magnesium nitrate, which makes it possible to electrically charge the phosphor particles and serve as a binder making it possible to obtain a highly adhesive coating. The organic solvent can be isopropanol.
This cataphoresis-based deposition procedure is not satisfactory in the case where the substrate has a transparent, electrically conductive indium and tin oxide film, because the latter is unable to withstand the deposition conditions, being reduced and consequently loses its transparency.
The present invention relates to a non-aqueous suspension for the deposition by cataphoresis of luminescent materials, particularly phosphors, which makes it possible to obviate this disadvantage.